Elongated heaters using metallic alloys as resistive elements are well-known. German patent document DE 29 17 639 describes an elongated electrical heater with an electrically insulating core. A resistive metallic element is wound around the core and to the resistive metallic element an insulating layer is applied.
The major drawback of this heater is the low reliability of the one-layer insulation. While bending and after long-term use, the resistive metallic element may press through the insulation.
European patent document EP 0 368 776 suggests an elongated heater, which contains one or more cores of a resistive element, on the outside coated with two electrically insulating layers. The outer layer is made of an inflexible insulating material preventing the electrical heater from bending. This is a major drawback, since the electrical heater's range of application is limited considerably, because it is not possible to adapt it to different dimensions of the objects to be heated.
Closest prior art, British patent document GB 1303917, which is chosen as the prototype, describes an elongated flexible heater containing a resistive element made of carbon fibers, realized with a cluster of filaments and with two insulating layers. The first layer permeates and secures the filaments of a resistive element and is made of an elastic heat-conducting electrically insulating polymeric material. The second layer is made of an analogous material in the form of a covering layer. Current supply tips are connected to the free ends of the resistive element. On the outside the unit is covered with an insulating tube. The drawback of this electrical heater is the low reliability of its insulation, since, in the process of permeating and securing the resistive element using an elastic insulating material, the coating's thickness may vary very strongly in respect of the bundle's perimeter and length, up to the point that in some parts there may be no insulation at all. In these parts there is only one insulation in the form of a covering layer. If this insulation is damaged, in particular because of the contact with the heated resistive element, the heater may be short-circuited and destroyed. Another drawback of this apparatus is the low reliability of the fastening of the resistive element to the current supply elements directly by the tip, since under pressure a strong contact between the fragile carbon fibers and the metal takes place. If such a contact takes place, a part of the carbon fibers will be destroyed which, under working conditions, will be followed by an increase of the contact resistance and by overheating and destruction of the contact point. If the heater is subjected to mechanical stress (tensile stress) the stress will be transmitted over the tip to the resistive element and there the heater might be destroyed because of weak mechanical contact with the carbon fiber. The problem resulting from the aforementioned is to overcome the above-indicated drawbacks and come to a technical result with a broader working temperature range, with an increased unit surface heat relase and with an improved reliability of the elongated flexible electrical heater and to present a simpler and more effective method of manufacturing it.